Showing papers by "Werner Wesch published in 2014"

Swift heavy ion irradiation of crystalline CdTe

Abstract: The influence of high electronic energy deposition in cadmium telluride (CdTe) was investigated using Rutherford backscattering spectrometry in channelling configuration as well as high-resolution transmission electron microscopy. Swift heavy ion irradiation was performed at room temperature with 185 MeV Au ions at perpendicular and non-perpendicular ion incidence. Independent of the ion incidence angle, neither ion beam induced point defects nor amorphous ion tracks were observed along the ion path. In contrast, the irradiated layer possesses a high crystalline quality even after irradiation with high ion fluences, i.e. multiple ion overlap. Nevertheless, irradiation with swift heavy ions leads to the formation of extended defects in a thin layer close to the sample surface. With increasing ion fluence the concentration of these extended defects increases continuously. This suggests that high electronic energy deposition causes the formation of defects, however, the combination of the high defect mobility within the thermal spike and the high ionicity of CdTe may benefit an effective recovery of the ionic bonds and consequently an easy recovery of the lattice, i.e. a nearly perfect recrystallization. The experimentally observed high defect resistivity enables a high doping of foreign atoms in CdTe over a wide depth range without the formation of lattice defects which is important for the use of CdTe as an effective absorber for solar cells.

Utilizing dynamic annealing during ion implantation: synthesis of silver nanoparticles in crystalline lithium niobate.

Abstract: Silver nanoparticles (NPs) embedded in lithium niobate were fabricated via ion beam synthesis and are suitable for various plasmonic applications, e.g. enhancement of optical nonlinear effects. After room temperature silver implantation, annealing in the temperature range of 400–600 °C was performed in order to recrystallize the damaged lithium niobate surface layer. The shape of the silver NPs, their optical properties as well as the structural properties of their surrounding matrix have been analyzed for various annealing steps. TEM investigations show that annealing at 400 °C does not lead to recrystallization of the damaged lithium niobate. A recrystallization occurs upon increasing the annealing temperature to 500 or 600 °C, but simultaneously a second phase consisting of lithium triniobate forms. This is additionally supported by XRD measurements. By utilizing dynamic annealing, i.e. implanting silver at elevated temperatures of 400 °C, it is shown that the LiNbO3 matrix stays single crystalline during ion implantation and no LiNb3O8 is formed. This is additionally verified by comparing the positions of the surface plasmon resonances with calculations based on Mie’s scattering theory.

Porous structure formation in ion irradiated germanium

Abstract: The ion beam induced modification of amorphous germanium is characterised by the formation of voids close to the sample surface and the transformation into a sponge-like porous surface layer at high ion fluences. This extreme structural modification of the sample surface is independent of the (heavy) ion species used and accompanied by a strong volume expansion. Nevertheless, recently it was demonstrated that buried voids (and buried sponge-like layers) can be formed in the depth of the projected ion range, however, only for the irradiation with I-ions at high ion fluences. Thus, the ion species and their chemical properties seem to play an important role in the structural modification around the projected ion range. In this paper we investigate the influence of the ion species on the ion beam induced void formation in Ge for room temperature irradiation with 380 keV I- and Au-ions as a function of the ion fluence. Independent of the ion species, a strong volume expansion is observed caused by void formation and the transformation into a sponge-like porous surface layer. For both ion species used, the final porous layers are structurally identical as established by cross section and plan view electron microscopy investigations. Further ion irradiation of the sponge-like porous structure, however, leads to significant differences in the ion beam induced structural evolution. For the Au-ion irradiation the porous layer remains nearly unchanged, whereas for the irradiation with I-ions a transformation from sponge-like to netlike porous layers occurs which is accompanied again by an extreme volume expansion. The underlying mechanism will be discussed based on chemical properties of the implanted ions.

Near-surface recrystallization of the amorphous implanted layer of ion implanted 6H-SiC

Abstract: The recrystallization and subsequent crystal growth during annealing of amorphous surface layers on 6H-SiC produced by ion implantation is investigated. Amorphous surface layers were produced by ion implantation of 360 keV ions of iodine, silver, xenon, cesium and strontium into single crystalline 6H-silicon carbide samples. The ion fluence for all the implantations were in the order of 10 16 cm −2 . Vacuum annealing of the damaged silicon carbide samples was then performed. The microstructure of SiC surfaces before and after annealing was investigated using a high resolution field emission scanning electron microscope (SEM). SEM analysis was complimented by Atomic Force Microscopy (AFM). SEM images acquired by an in-lens detector using an accelerating voltage of 2 kV show nano-crystallites developed for all implanted samples after annealing. Larger and more faceted crystallites along with elongated thin crystallites were observed for iodine and xenon implanted 6H-SiC. Crystallites formed on surfaces implanted with strontium and cesium were smaller and less faceted. Strontium, silver and cesium implanted samples also exhibited more cavities on the surface. AFM was used to evaluate the effect of annealing on the surface roughness. For all the amorphous surfaces which were essentially featureless, the root mean square (rms) roughness was approximately 1 nm. The roughness increased to approximately 17 nm for the iodine implanted sample after annealing with the surface roughness below this value for all the other samples. AFM also showed that the largest crystals grew to heights of about 17, 20, 45, 50 and 65 nm for Sr, Cs, Ag, Xe and I implanted samples after annealing at 1200 °C for 5 h respectively. SEM images and AFM analysis suggest that iodine is more effective in promoting crystal growth during the annealing of bombardment-induced amorphous SiC layers than the rest of the ions we implanted. In samples of silicon carbide co-implanted with iodine and silver, few cavities were visible on the surface indicating that iodine influenced the recrystallization. Surface crystallites that grew on the iodine implanted surfaces were more resistant to thermal etching and decomposition than those that grew on the silver implanted samples at temperatures of up to 1400 °C for 30 h.

Ion-beam-induced thin film stress in lithium niobate

Abstract: The dominating modification of crystalline solids by energetic ions is the formation of lattice defects, which accumulate with ongoing irradiation. Many materials exhibit a phase transition from crystalline to the amorphous state at higher ion fluence. However, this ion-beam-induced structural modification involves the formation of mechanical stress, which is generally disadvantageous for the successful application of ion irradiation in the micro-device technology. Hence, a fundamental understanding of the ion-beam-induced stress evolution is crucial for the effective use of ion beam technology. Lithium niobate (LiNbO3) is a promising candidate for the application of integrated photonic structures. However, for the fabrication of such structures in LiNbO3 ion irradiation is indispensable. In order to get a fundamental and comprehensive understanding of the ion-beam-induced defect and stress evolution in LiNbO3, irradiations with varying parameters (ion energy and irradiation temperature) over a wide range of ion fluence for different crystallographic orientations were performed. The ion-beam-induced defect and stress evolution were studied by means of in situ Rutherford backscattering spectrometry and laser reflection technique, respectively.The investigations demonstrate that ion-beam-induced defect and stress evolution in LiNbO3 is highly anisotropic. Moreover, a complex stress evolution is observed, i.e. with increasing ion fluence different stress formation and stress relaxation processes occur. It will be demonstrated that effects such as radiation-induced viscosity or anisotropic deformation that were suggested by previous models cannot explain the stress evolution in LiNbO3.This work presents a new approach that describes the anisotropic stress and defect evolution in LiNbO3 by a complex defect formation mechanism, i.e. the presence of different defect types and their transformation into each other. Each defect type strains the surrounding crystal matrix and leads to a macroscopic deformation. The total stress is the superposition of the individual stress caused by different defect types.

Structural and Luminescent Properties of Sn-Doped SiO 2 Layers

Abstract: The formation of tin nanocrystallites in a SiO2:Sn matrix using a high-dose implantation technique followed by high-temperature processing was studied. Structural phase transformations were studied by plan-view transmission electron microscopy. Optical properties of the implanted samples were investigated by photoluminescence. It was shown that annealing of the implanted SiO2 layers formed nanoprecipitates of β-Sn and caused the appearance of regions enriched in SnO2. Photoluminescence spectra of implanted and annealed samples exhibited intense emission in photon energy range 1.3–3.6 eV that was attributed to oxygen-deficit centers created in the SiO2:Sn matrix and at the nanocluster/SiO2 interface.